Low pressure steam turbines typically utilize a central inlet portion through which steam is initially passed and divided between sets of turbine blades. Referring to FIG. 1 a typical prior art low-pressure steam turbine 10 is shown. Steam from a source (not shown) is provided to the turbine 10 through a conduit 12 which passes through the outer casing 14, and attaches to the inner casing 16. Steam passes through an opening in the outer casing 14, through an opening in an inner casing 16, and then to an inlet chamber 18, which is formed within the inner casing 16. A rotor 20 is mounted by a bearing 22 at each end; the rotor 20 rotates about an axis of rotation "A." Annular rows of blades 24 are disposed along the rotor 20. A number of stationary annular rows of blades 26 are operatively positioned in relation to the rotor blades 24 for directing steam to the rotor blades 24. The stationary blades 26 are positioned through their attachment to various blade rings, which in turn are attached to the walls 30 of the inner casing 16. Inlet blade ring sections 29a,29b are positioned such that an opening 31 is formed therebetween for the passage of the flow of steam. The inlet blade ring sections 29a,29b are disposed so as to circumferentially surround the blades nearest the inlet. The inner casing 16 is aligned to outer casing 14 by fitted dowel assemblies 32.
The inlet chamber 18 is shown to include sidewalls 34 which are oriented at an angle to the axis of rotation "A." Sidewalls 34 are attached at one end to the walls 30 and at the other end to the inlet ring sections 29a,29b. A number of staybars 36, of which only one is shown, are provided between the inlet ring sections 29a,29b in order to join each pair of inlet ring sections 29a,29b. been used. The inlet ring sections 29a,29b were either comprised of rolled steel half-rings to which the staybars 36 were welded, or the inlet ring sections 29a,29b were cast assemblies with integral bosses to which the staybars 36 were welded. Finally, a number of ribs 38a,38b are positioned within the inner casing 16 about the rotor 20, such that the ends of each rib 38 is in contact with the walls 30 and the inlet ring sections 29a,29b.
In operation, steam is supplied to the turbine 10 through the conduit 12, passing through the outer and inner casings 14,16 to the inlet chamber 18. Inlet chamber 18 directs the flow of steam to the midpoint of the rotor 20 where the steam expands axially through alternating annular rows of stationary blades 26 and rotational blades 24 thereby causing rotation. After crossing the last row of blades, the flow of steam is directed through an exhaust 39 and exits the turbine 10.
As explained above, the inlet ring sections 29a,29b are typically joined by staybars 36. In most constructions, the rings themselves are split half-rings or semicircular sections which are joined by staybars to form inlet ring subassemblies. As known to those of ordinary skill in the art, the inlet ring subassemblies are typically joined by a horizontal joint. In a case where the inlet ring sections are rolled from pressure vessel steel plate, the cross-section is rectangular. This type of design creates an obstruction in the steam path that disturbs the flow. Moreover, the staybars themselves are welded to the rings which are in turn welded to the inner cylinder assembly. These extensive welds provide numerous stress concentration sites and frequently result in fatigue failure. Also, the plate from which the ring is formed could delaminate when it is rolled into an inlet ring section or during the welding operations, thereby causing a decrease in the material strength and necessitating repair. This type of failure is known as "laminar tearing." On the other hand, in the case of cast inlet ring sections, a desirable triangular cross-section may be obtained which better directs the steam flow. Cast inlet ring sections, however, also have integral cast bosses, which provide attachment locations for the staybars. The pairs of inlet ring sections are therefore still joined axially to form inlet ring subassemblies by welded-in staybars, typically of cylindrical cross-section. These staybars require extensive weld preparations and massive welds in order to hold them in place. The welds present exposed, irregular surfaces that produce fatigue-prone sites due to the high stress concentrations created.
It is therefore an object of the present invention to eliminate the weld preparation and welds related to the joining of cast inlet ring sections into inlet ring subassemblies which are prone to fatigue-induced failure.
It is a further object of the present invention to eliminate the use of rolled steel inlet ring sections and the welds used therein, thereby eliminating laminar tearing and other delamination failures in the inlet ring sections.